The present invention is directed to a motor-driven hand tool, such as a drill, a hammer drill, a grinder or the like, and includes a housing containing a drive motor. A shaft driven by the drive motor is located in and projects out of the housing for transmitting rotary motion to a tool bit secured on the shaft. Further, an inert mass is rotatably supported in the housing coaxial with the driven shaft and connected to the housing by a spring element.
Hand tools of the above type are used principally by craftsmen, however, in increasing numbers, they are also used by do-it-yourselfers. There is the danger in using such tools that the tool bit jams or becomes seized and the driven shaft is blocked, with the result, that as a reaction, the housing rotates about the axis of the driven shaft. If the tool operator does not anticipate such a reaction, the hand tool can be thrust out of the operator's hands. If he is working in a dangerous position, such as on scaffolding or a ladder, he can lose his balance and fall to the ground. The blocking of the tool can, in addition, tend to overstress the joints of the operator. To avoid such situations, it has been known in DE-OS No. 2 326 087 to arrange an overload clutch between the drive motor and the driven shaft so that disengagement takes place if a predetermined torque is exceeded. When the overload clutch interrupts the drive between the motor and the driven shaft, the operator must supply a correspondingly large reaction torque. If the operator is unable to provide the reaction torque, then there is no interruption of the drive and the tool remains operative.
In a further solution disclosed in DE-OS No. 3 041 099, an inert mass is rotatably supported in the housing coaxial with the driven shaft. If the tool bit becomes seized, the housing rotates around the receiving spindle, however, the rotatably supported inert mass remains immobile because of its inertia and a relative rotation takes place between the housing and the inert mass. Preferably, such relative rotation is determined by a contactless sensor and a signal is produced and utilized for switching off the energy supplied to the tool or for actuating a clutch or an electromechanical brake. To return the inert mass into its original position, the mass is connected to the housing by a spring element. This arrangement requires expensive electronic or electromechanical components, and such components are sensitive and are marginally usable due to the rough treatment experienced by the hand tool on a construction site.